The present invention relates to a taking optical system for a camera. More particularly, the present invention relates to a taking optical system suitable for a camera which is capable of both ordinary photography and trimming photography.
Trimming photography is a photographic method wherein, as shown in FIG. 10(a), a predetermined area B in an ordinary recording area A of a film 50 is enlarged during printing process and printed on a photographic paper 52 of ordinary size as shown in FIG. 10(b), thereby enlarging an image of the region B to obtain a pseudo image which looks as if it had been taken with a telephoto lens having a longer focal length than that of the lens actually used to take the photograph. A typical trimming camera having a trimming system has a device for recording trimming information on a predetermined position 51 of a film or a film cartridge so that a printer can automatically recognize the trimming information from the film used for trimming photography and enlarge the image area B to an appropriate size during printing process. The trimming camera may additionally have a function which enables the user (photographer) to recognize the image area B used for trimming photography through the view finder.
The main trimming information includes the position and size of the trimmed image area. As the trimming size, the ratio of the ordinary recording area A to the trimmed image area B on the film may be used, and this ratio will hereinafter be referred to as trimming magnification. For example, the ratio of the vertical length 1.sub.O of the ordinary recording area A, shown in FIG. 10(a), to the vertical length 1.sub.T of the trimmed image area B, i.e., l.sub.O /l.sub.T, may be called trimming magnification.
In such trimming photography, an image frame can readily be enlarged. However, since the image frame is enlarged by printing process, the image quality, particularly the resolving power, becomes degraded on the resulting print. Roughly speaking, since the image frame is enlarged more than for ordinary photography by the square of the trimming magnification, the resolving power on the print lowers in proportion to the reciprocal of the square of the trimming magnification.
Trimming information writable cameras, in which information indicating a set enlargement area (trimmed image area B) is recorded on the film, have heretofore been proposed in, for example, Japanese Patent Application Laid-Open (KOKAI) Nos. 54-26721, 62-52541, and so forth.
As has been described above, in trimming photography a relatively narrow area on a negative film is enlarged to a predetermined area on a print, and hence the ratio of enlargement in the printing process is relatively high. Accordingly, cameras used for trimming photography must be improved in the performance of the taking lens.
More specifically, if a taking lens has MTF (Modulation Transfer Function) as shown in FIG. 11(a) for ordinary photography, it must have MTF as shown in FIG. 11(b) to obtain a print of the same MTF by trimming photography. This will readily be understood by taking into consideration the difference between the enlargement ratio in the printing process for ordinary photography, shown in FIG. 12(a), and that for trimming photography, shown in FIG. 12(b). It should be noted that in FIGS. 11 and 12 the trimming magnification is assumed to be 2.
Incidentally, Japanese Patent Application Laid-Open (KOKAI) Nos. 63-285524, 04-97326 and 04-97325 disclose techniques whereby, in a single-focus lens of a camera capable of trimming photography, a lens spacing is varied according to trimming to correct the curvature of field, thereby improving the lens performance for the edge area of the image frame.
Japanese Patent Application Laid-Open (KOKAI) No.61-123811 discloses an example of a technique whereby the lens performance for the edge area of the image frame is improved by floating according to the change of field angle, although this is not concerned with trimming. In this example, astigmatism and coma are corrected by varying the spacing of afocal lens elements which are unrelated to zooming, thereby improving the lens performance for the edge area of the image frame.
Meantime, U.S. Pat. No. 5,070,350 discloses an apparatus designed so that since the position of an average optimal image plane of a lens having a field curvature changes according to trimming, the position of the film plane is corrected to the position of the average optimal image plane according to trimming, thereby improving the lens performance for the edge area of the image frame.
On the other hand, Japanese Patent Application Laid-Open (KOKAI) No. 55-36833 discloses a technique of improving the lens performance for the central area of the image frame according to zooming. However, this technique aims at improving the lens performance for the central area of the image frame in the case of macro photography at the long-focus side of zooming and is therefore unrelated to photographic techniques other than macro photography.
The conventional methods of maintaining the desired image quality by improving the off-axis performance according to trimming are effective for lenses which are originally inferior in performance, for example, lenses with which aberrations such as field curvature and coma aberration reach a maximum at an intermediate image height, but not effective for lenses with which off-axis aberrations are gradually aggravated as the image height increases. That is, in Japanese Patent Application Laid-Open (KOKAI) Nos. 63-285524, 04-97326 and 04-97325, although the lens performance for the edge area of an image frame on a negative film improves, the lens performance for the central area of the image frame does not improve. Therefore, as the trimming magnification increases, the lens performance for the central area steadily lowers.
In addition, U.S. Pat. No. 5,070,350 suffers from the problem stated below. In a lens having a large field curvature, the average optimal image plane and the optimal image point in the center approach each other as the trimming magnification increases, and thus the lowering of the lens performance for the central area is reduced. However, since the lens performance for the central area is originally inferior, excellent image quality cannot be obtained after all.
Although Japanese Patent Application Laid-Open (KOKAI) No. 55-36833 discloses a technique whereby the lens performance for the central area is positively improved, this technique aims at improving the lens performance for the central area in the case of macro photography at the long-focus side of zooming and is therefore unrelated to photographic techniques other than macro photography, as described above.